In neurodegenerative brain disorders, aggregates of misfolded protein oligomers can undergo neuron-to- neuron transfer. This results in the seeded assembly of misfolded host proteins in recipient cells, thus propagating the pathogenic protein species. Recent progress has demonstrated that in Parkinson?s disease (PD), ?-synuclein forms pathogenic fibrils that can spread between interconnected neurons across synapses and brain regions. This may contribute to the dementia that develops in PD patients over time. It is unlikely that pathogenic ?-synuclein assemblies have a single neuronal ?receptor?. Indeed, the first few partners of ?- synuclein fibrils are beginning to be identified. However, the pathophysiological interactions of ?-synuclein fibrils with neuronal and synaptic plasma membrane proteins remain unclear, and how their ?-synuclein fibril retention impacts synaptic properties has not yet been addressed. Our central hypothesis is that synaptic adhesion molecules can contribute to ?-synuclein spread due to their cellular location and the synaptic pathology in PD. Our preliminary data on pathological effects of ?-synuclein fibrils support this hypothesis. In addition, we aim to establish an unbiased proteomic approach to screen for neuronal surface partners of ?- synuclein fibrils. Two specific aims will be pursued to test our central hypothesis. First, we will determine the pathophysiological effects of ?-synuclein fibril binding on the load of aggregated endogenous ?-synuclein in recipient cells and on their synapses. Second, we will establish and employ an innovative proteomic approach to identify the complement of neuronal membrane proteins that are targeted by ?-synuclein fibrils. Our approaches include an assay using preformed ?-synuclein fibrils that we have established for application in cultured primary neurons, including microfluidic chambers and assays of synapse assembly, together with a proteomic labeling approach we have established. We anticipate first, to define how synaptic target interactions with ?-synuclein fibrils impact ?-synuclein transmission and synaptic properties, and second, to perform a proteomic screen to identify the complement of ?-synuclein fibril partners in neuronal membranes. This approach will be applicable to multiple types of neurons and can hence advance the field by identifying cell- autonomous factors that affect neuronal vulnerability across distinct regions. The expected progress is significant because it will define the roles of synaptic targets in PD-relevant synaptic pathophysiology and enable identifying therapeutic targets across neuron types. Together, this project can reveal mechanisms underlying ?-synuclein spread and PD progression.